Chlorine dioxide used as an aqueous solution is of considerable commercial interest and importance, mainly in the area of pulp bleaching but also in water purification, fat bleaching, removal of phenols from industrial wastes, etc. It is therefore desirable to provide processes by which the chlorine dioxide can be efficiently produced.
The predominant chemical reaction involved in such processes is summarized by the formula EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O (1)
The chlorate ions are provided by alkali metal chlorate, preferably sodium chlorate, the chloride ions by alkali metal chloride, preferably sodium chloride, or by hydrogen chloride, and the hydrogen ions by mineral acids, normally sulfuric acid and/or hydrochloric acid.
There are also competing reactions which decrease the efficiency of the formation of chlorine dioxide from chlorate ions, the main reaction being represented by the formula EQU ClO.sub.3.sup.- +6H.sup.+ +5Cl.sup.- .fwdarw.3 Cl.sub.2 +3H.sub.2 O (2)
The efficiency of commercial processes, such as the SVP process (SVP is a registered Trade Mark owned by KemaNord, Sweden) are normally above 90% and preferably above 95%, which means that the competing reaction represented by formula (2) is very much suppressed.
Processes for producing chlorine dioxide are set forth in e.g. U.S. Pat. No. 3,563,702 and 3,864,456, which are hereby incorporated by reference, comprising continuously feeding to a single vessel generator-evaporator-crystallizer alkali metal chlorate, an alkali metal chloride and mineral acid solutions in proportions sufficient to generate chlorine dioxide and chlorine, at a temperature of from about 50 to about 100 degrees centigrade, and an acidity of from about 2 to about 12 normal, with or without a catalyst, removing water by vacuum-induced evaporation at about 100-400 millimeters of mercury absolute, with concurrent withdrawal of chlorine dioxide and chlorine, crystallizing the salt of the mineral acid within the generator and withdrawing the crystals from the vessel.
In those reaction systems wherein the acid normality is maintained between about 2 and 4.8, the reaction may be carried out in the presence of a relatively small amount of a catalyst, such as those selected from the group consisting of vanadium pentoxide, silver ions, manganese ions, dichromate ions and arsenic ions.
As the reaction occurs within the generator in producing chlorine dioxide from sodium chlorate and sodium chloride, where sulfuric acid is employed as a mineral acid reactant, crystals of sodium sulfate are crystallized and withdrawn in the form of a slurry. The main reaction is shown by the formula EQU NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +Na.sub.2 SO.sub.4 +H.sub.2 O (3)
Sodium sulfate is a valuable by-product useful in kraft pulping operation. It is used in the chemical recovery system in order to cover losses of sulfur and sodium.
In some cases, however, the requirement for sodium sulfate is greatly reduced or obviated. In certain kraft mill operations, the requirements for sodium sulfate may be reduced or varied. While the requirement for reduced quantities of sodium sulfate may vary, the requirement for the chlorine dioxide remains or even increases.
In order to reduce the quantities of sodium sulfate it has been suggested in U.S. Pat. No. 3,933,987 to substitute part of the sodium chloride with hydrochloric acid as a source for the reducing agent. Here, the hydrochloric acid serves the dual function as a reducing agent and as a portion of the mineral acid. By using the combination of sulfuric acid and hydrochloric acid the amount of precipitated sodium sulfate can be effectively controlled and reduced. When one mole of hydrochloric acid and half a mole of sulfuric acid are used, the amount of precipitated sodium sulfate is reduced by 50% compared to the mode when all chloride ions are added as sodium chloride, as can be seen by comparing formula (3) with the following formula: EQU NaClO.sub.3 +HCl+1/2H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +1/2Na.sub.2 SO.sub.4 +H.sub.2 O (4)
The main reactions for the chlorine dioxide production always also produce half a mole of chlorine per mole of chlorine dioxide, see formulas (1), (3) and (4). An additional small amount of chlorine is also produced by the side reaction according to formula (2). This chlorine product has formerly been used as such in the paper mills as a bleaching agent in aqueous solution, or it has been reacted with sodium hydroxide to form sodium hypochlorite and used as a bleaching agent. Today there is a tendency towards a more extensive chlorine dioxide bleaching and thus a decreasing need for chlorine and hypochlorite as bleaching agents. Another disadvantage with by-product chlorine is that the concentration of the chlorine solution is very low, normally 1-5 g/l. Because of the large amount of water, modern bleaching systems cannot use the by-product chlorine in the chlorination stage. This means that many pulp mills consider the chlorine as a by-product of limited value.
In order to make the chlorine dioxide process more attractive, it has been suggested that the amount of chlorine leaving the plant can be reduced by reacting the by-product chlorine with sulfur dioxide and producing a mixture of sulfuric acid and hydronchloric acid according to the formula EQU Cl.sub.2 +SO.sub.2 +2H.sub.2 O.fwdarw.2HCl+H.sub.2 SO.sub.4 ( 5)
The produced mixed acid can thereafter be used directly as acid feed to the chlorine dioxide reactor operating in a partial hydrochloric acid mode according to formula (4).
Such processes have been previously described in U.S. Pat. No. 3,347,628 and Swedish published patent application No. 8103892. In U.S. Pat. No. 4,086,329 a balanced process is described, where just sufficient sulfuric acid is produced according to formula (5) in order to get the exact need of sulfate ions to be combined with the sodium ions introduced to the chlorine dioxide generating system with the sodium chlorate.
The reaction of sulfur dioxide and chlorine obtained in the chlorine dioxide generating process is also disclosed in U.S. Pat. No. 4,393,036. Here the reaction is effected in an aqueous medium at a temperature below the boiling point of water. In order to accomplish this, the reaction is carried out in a cooled falling film tower having integral cooling passages or in a packed bed tower with a substantial portion of the produced mixed acid being recycled through an external heat exchanger. This patent shows the difficulties in controlling the exothermic reaction of chlorine and sulfur dioxide.
Another difficulty in returning the mixed acid produced by reacting by-product chlorine with sulfur dioxide, is that the mixed acid normally contains unreacted sulfur dioxide which might be detrimental to the chlorine dioxide reaction when the mixed acid is recycled to the reactor. Sulphur dioxide is a reducing agent which can interact with the chloride ion as reducing agent in the reaction and thus leads to imbalance in the reaction and to a decrease in efficiency. It has also been observed that sulfur dioxide dissolved in the mixed acid forwarded to the chlorine dioxide reactor might have a detrimental effect on the morphology of the sodium sulfate crystals precipitated in the process. This is due to the reducing effect of sulfur dioxide on dichromate ions. The dichromate ions can be present in the cell liquor from a chlorate cell, which cell liquor is used as the source of sodium chlorate feed to the reactor. These dichromate ions are reduced to trivalent chromium which causes the sodium sulfate to precipitate as very fine crystals which might be difficult to separate from the reaction medium. In order to avoid sulfur dioxide in the acid solution, it is suggested in the U.S. Pat. No. 4,393,036 to effect the reaction of chlorine and sulfur dioxide in a great excess of chlorine.